The invention relates to a cold crucible system for melting and crystallizing non-metallic inorganic compounds having a cooled crucible wall in the form of metal pipes through which cooling medium flows and which are in mechanical connection with the crucible bottom through which cooling medium also flows and having a first induction coil which surrounds the crucible wall and via which high-frequency energy can be coupled into the contents of the crucible, as well as a second induction coil which can be switched independently of the induction coil surrounding the wall of the crucible and which is arranged below the crucible bottom.
The invention further relates to the use of the cold crucible system for growing single crystals from melts on the basis of a rare earth metal/gallium garnet.
A cold crucible system is suitable for carrying out the so-called skull melting method for melting starting materials for the growth of single crystals by direct coupling-in of an electromagnetic field from an induction coil into the material to be heated. Such a crucible may be formed having a wall of water-cooled copper pipes which are arranged closely beside each other in the form of a circle while the bottom of the crucible may consist of a water-cooled metal plate or of several copper pipes, a high-frequency coil being provided around the cylindrical crucible wall. The electromagnetic field penetrates through the slots between the copper pipes into the interior of the crucible. Such a crucible is known, for example, from V. I. Aleksandrov, V. V. Osiko, A. M. Prokhorov and V. M. Tatarintsev "Synthesis and Crystal Growth of Refractory Materials by RF Melting in a Cold Container" in: Current Topics in Materials Science, Vol. 1, ed. by E. Kaldis, North Holland Publ. Comp., 1978.
The operation of such a known cold crucible will be described with reference to an example. For melting, the material is generally filled in the crucible in powder form. Poorly conducting oxides must first be preheated. For this purpose, pieces of metal which correspond to the oxide to be melted are used which are embedded in the powder. The electromagnetic field first heats the pieces of metal through the induced eddy currents which in turn melt the oxide powder in the immediate proximity of the metal pieces. The the high frequency field of the coil can couple-in directly in the melt being formed due to the higher electric conductivity of the melt. By increasing the high-frequency power addition oxide powder is continuously melted until the melt comes in the proximity of the crucible wall. The water-cooled inner crucible surface ensures that between the surface of the crucible wall and the hot melt a densely sintered specific layer which is present in a solid phase is formed which protects the crucible from the attack by the melt. The metal used for preheating is converted into the oxide to be melted by oxygen from the air.
For example, Al.sub.2 O.sub.3 single crystals have been grown by means of such known crucibles. For this purpose, an Al.sub.2 O.sub.3 melt has been seeded with a seed crystal of sapphire. By drawing the seed crystal away from the surface of the melt at a rate of from 10 to 30 mm/h, Al.sub.2 O.sub.3 single crystals could be manufactured in a length up to 160 mm and a diameter up to 35 mm.
It has now been found that perfect crystals can be grown by means of the known crucible only in exceptional cases. Difficulties may present themselves in particular in melts of low viscosity. For example, turbulent flux lines can be observed at the surface of the melts which indicate that turbulent convection currents are induced as a phenomenon resulting from the absorption of the high-frequency field in the melt. A radial temperature distribution on the surface of the melt with a temperature minimum in the centre, as it is required for growing single crystals, cannot adjust in such a melt. When the temperature of the melt is reduced for the purpose of increasing the crystal diameter of the crystal to be grown, the crystal seeds simultaneously also grow on the sintered layer present in a solid phase on the cooled crucible inner wall and the cooled crucible inner bottom. By increasing the thickness of said layer the volume of the melt is reduced and hence its electrical conductivity. The electric power no longer couples in sufficiently so that the temperature of the melt decreases further. This process occurs exponentially acceleratedly so that also when the high-frequency power is increased, the melt reaches a critical volume after a short time and solidifies.
The thermal losses at the cooled inner surfaces of the wall of the crucible and the necessity to operate as far as possible above the critical melt volume (thin sintering layer) necessitate a considerable overheating of the melt. Resulting phenomena of this overheating are the evaporation of volatile material components and turbulent melt convections coupled with temperature fluctuations.